This invention relates to a system of retransmitting data upon occurrence of errors in a data transmitting device. More particularly, it relates to a data retransmitting system which, upon occurrence of errors in data transmission, can effectively retransmit the data irrespective of transmission distance.
In data transmission over communication lines, data errors occur because of noise in the communication lines or the like. When such an error occurs, its effect is increased as the data compression rate increases. In general, a high speed data transmitting system cannot sufficiently deal with the effects of such errors. Accordingly, a system is generally employed in which, when errors occur, they are corrected by retransmitting the data.
For instance, in a digital facsimile system using telephone networks, data to be transmitted is divided into frames, which are successively transmitted. In such a system, the data of each frame is checked on the signal receiving side, so that, when the error is higher than a reference value, a "negative acknowledge" tone for correcting the error is transmitted to the signal transmitting side. When the error is lower than the reference value, an "acknowledge" tone is transmitted to the signal transmitting side. These replies are generally sent to the signal transmitting side before the next frame is transmitted. In this case, the length of each frame (hereinafter referred to as "a frame length", when applicable) should be longer than a transmission delay time between signal transmission and reception. Accordingly, in retransmission of data, the frame length is unreasonably long, and the overall time required for communication is increased.